WX BRIEF

Precipitation precautions

There's more to rain and snow than just water falling from the sky.


(Top)Penny-sized hail coats the ground after a June thunderstorm. Hail is a form of frozen precipitation that occurs when an ice embryo is carried aloft in a thunderstorm either once or many times, and is coated each time with a glaze layer from supercooled droplets it encounters. Hailstones can reach several inches in diameter and may weigh over 2 lbs. Snow pellets accumulate in a rain gauge after a brief November shower. Snow pellets are smaller versions of graupel and form as supercooled liquid freezes to an ice crystal.

(bottom)This allows the droplets to continue their preferential attraction of water vapor at lower humidity levels. In addition, a solution makes it more difficult for the water to evaporate from the droplet. These features are critically important as they are offset against the evaporative effects of the curved surface of the droplet.

The curve of the droplet's surface promotes evaporation. Smaller droplets have a greater surface curvature, meaning a higher evaporation rate. The result is that, for very small droplets, the air immediately surrounding the drop is unsaturated, even though the atmosphere in which the drop sits may be saturated.

It is up to the solute effect to counter this evaporation. It is able to do this because the hygroscopic solution achieves saturation with respect to its surroundings at relative humidities as low as 70%. Thus, the higher water vapor content of the atmosphere means the air immediately around the droplet is supersaturated. In general, this supersaturation humidity is less than 101%.

But supersaturation allows the droplets to continue to grow, reducing the supersaturation, until they reach cloud droplet status (0.1-0.2 mm), at which point their reduced curvature and weakened solution means they achieve an equilibrium with their surroundings and no longer grow. This is the main reason why the majority of clouds exist without ever producing a raindrop.

Terminal velocity

As soon as a cluster of molecules becomes a droplet, it has a mass that is greater than what could be suspended merely by the balance between gravity and pressure forces.

The droplet will want to fall toward earth. Fortunately, since cloud droplets have such a small mass, their terminal velocity will normally be countered easily by microscale upward air currents. Cloud droplets that do rise or fall are likely to evaporate quickly as they enter a new moisture regime, which is why most clouds are limited to a narrow altitude zone.

The movement of cloud droplets is largely responsible for their growth into raindrops. As they move, some collide with each other and merge to become larger droplets. With increased mass, they may be able to overcome any upward air currents and fall, colliding with even more cloud droplets on the way down and increasing their size and speed.

The amount of time a rain droplet spends moving through a cloud will usually determine its exit size. If a raindrop spends only moments falling out of a shallow nimbostratus, it may be only 0.5 mm in diameter, forming part of a light drizzle.

Conversely, a droplet that has been shot aloft by a strong thunderstorm updraft and then falls 30,000 ft before exiting the cloud base may have grown to 4 or 5 mm in diameter. The average raindrop has a diameter of around 1 to 2 mm.

Recalling that smaller droplets with greater curvature have greater surface evaporation, we can expect that most light drizzle droplets will evaporate soon after exiting the clouds-and, in fact, unless the air through which drizzle falls has a humidity above about 90%, most of the rain won't reach the ground.

Liquid vs solid

Despite the disruptions it causes, snow is actually a relatively rare form of precipitation. Considering that only certain impurities can act as ice crystal nuclei, that air temperature aloft must be below freezing, and that even at -10 degrees C (14 degrees F) liquid cloud droplets outnumber ice crystals by 1 million to 1, it seems a wonder that it snows as often as it does.

Meteorologists use computer models of the atmosphere to help them determine where, based on the physics of the atmosphere, precipitation is most likely to occur. In this image of precipitation probability for the subsequent 12 hours, darker green shading indicates higher chances of precipitation, although precipitation type is not indicated.

Furthermore, if snow encounters above-freezing temperatures on its way to the surface, some melt will take place and, depending on its ability to refreeze, the snow can quickly transform into ice pellets, freezing rain or just regular rain.

Given the relative shortage of ice crystals, how can some places get over a meter of snow in a single storm? Like liquid precipitation, water vapor can be deposited on a nucleus (for ice it's known as a freezing nucleus).

Unlike liquid, however, there is no solution effect, because the impurity is not dissolved by the ice. But an undissolved nucleus in a liquid droplet can trigger instantaneous freezing of the liquid around itself when its critical subfreezing temperature is reached.

At very low temperatures, a liquid droplet may freeze spontaneously without any assistance from a nucleus. The small ice crystal is known as an ice embryo, because it forms the core of the future snowflake.

The disparity between the number of ice crystals and liquid droplets in a cloud at most air temperatures means that the ice crystals need to have some way of preferentially attracting moisture.

This is accomplished because of the difference in energy needed to evaporate a molecule from water (termed latent heat of vaporization) and the energy used to sublimate a molecule from ice (latent heat of sublimation).

While the difference is not great, it means a difference in the amount of vapor needed for saturation and, for temperatures from 0degrees to -40degreesC, air that is saturated for water is supersaturated with respect to ice.

The difference in saturation levels means that, while both crystals and droplets may grow, the crystals will grow at a faster rate. Eventually, enough water is removed from the air that the droplets will start to evaporate to maintain a balance, but the ice crystals can continue to grow.

This crystal growth at the expense of the liquid droplets is known as the Wegener-Bergeron-Findeisen process, and it operates best at temperatures between -8 and -16 degrees C. The process is most effective at scavenging from the liquid droplets when there are between 1 and 10 crystals per million liquid droplets.

 

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